The invention relates to a signal treating circuit with a class A/B output stage connectable to a source of multiple supply voltages, with means for connecting the output stage again and again to a one of said multiple supply voltages as a function of a voltage value of a signal treated by the signal treating circuit.
A signal treating circuit to which the invention relates may be for example a driver for telephones. The output stage of a driver for telephones operates mostly in class-A/B mode with a small bias current of about 100 uA and is capable to deliver currents of 20 mA or more to the speaker. The audio signal to be delivered is on average at a level of about 10% of full output level. The output stage operates in a balanced (bridge) driver configuration.
The value of the supply voltage is determined by the maximum voltage swing of a load. As a consequence at low signal levels the voltage across the output transistors is much larger than the voltage across the load. Consequently power dissipated in the output stage is about four times larger than the power delivered to the load.
It is an object of the present invention to provide a method and a signal treating circuit that operates more efficiently with the power supplied by a power supply to the signal treating circuit and the output stage.
A signal treating circuit according to the invention thereto is characterized in that the function is a function of a first instantaneous voltage value of the signal as treated by the signal treating circuit.
Thereby it is achieved that actually when a larger signal is present the switch is made to a higher voltage as a supply voltage for the output stage. This is in contrast to methods and circuits in which an average level of the signal over a certain larger period of time is taken as a measurement of the presence and absence of smaller and larger output signals.
In a situation that more than one supply voltage is available the output stage can operate on a lower supply voltage, for example 0.9 V, for small output signals and switch over to a higher voltage, for example 2 V, for larger signals. Also the standby current, i.e. the current through the output stage of a class-A/B mode output stage is in such a situation delivered by the lowest supply voltage. A DC operating level at the output of a class-A/B mode output stage in the above example is then 0.45 V for small output signals and 1 V for larger output signals.
The signal, a voltage value of which is used to determine to which supply voltage the output stage will be connected may be the signal at an input of the signal treating circuit, the signal somewhere in the signal treating circuit between an input and the output stage or it may be the signal as treated by the signal treating circuit at the output stage.
A preferred embodiment of a signal treating circuit according to the invention is characterized by means for generating at least one reference voltage value, a memory for storing voltage values and by means for storing at least a second instantaneous voltage value of the signal in the memory when the first instantaneous voltage value of the signal crosses one of said at least one reference voltage values.
Thereby it is achieved that voltage values of the signal can be easily compared to the stored values. This is of particular advantage when the voltage value of the signal is present in digital form and the stored voltage value of the signal also is available in digital form. In such a case a simple digital comparison can be made to determine whether or not an actual value of the voltage value of the signal is above or below a stored voltage value of the signal.
It is to be noted that the invention is independent of the type of signal, analog or digital.
A still further preferred embodiment of the invention is characterized in that the signal treating circuit comprises a D/A converter and a non-linear operating circuit between an input of the signal treating circuit and the D/A converter, in that in operation the non-linear operating circuit generates a jump in DC level at an output of the output stage and in that means are present for at the same point of time making jumps in DC level at the output of the output stage and making changes in supply voltage for the output stage.
A still further preferred embodiment of a signal treating circuit according to the invention in which the output stage comprises first and second output terminals and the first and second output terminals are provided with the signal as treated in opposite phases is characterized by means for raising the signal as treated at the first output terminal when the signal as treated at the second output terminal clips and vice versa and by means for raising during clipping the voltage value of the signal as treated that does not clip with an extra amount that compensates for the clipping of the signal as treated that clips.